I am doing the eyeball tutorial from simplymaya right now and I am at the point at which I have to assign materials to the eyeballs. However it seems that I cannot assign any material to my objects. However, when I create a new polygon sphere I can assign materials to that object.

When I reopen an older savefile I can assign a material to my objects before the UV-mapping part. So I guess it has something to do with that.

Does anyone know what is going wrong here?